This invention relates to a greenhouse device or complex of a relatively large scale that may be used especially in a frigid district where the outdoor temperature during winter may fall to -45.degree. C. or even lower.
Heretofore, in using a glass greenhouse for growing of fruits, vegetables or flowers in such cold climate, the temperature difference between the inside and outside of the greenhouse may increase to that much during winter. Moreover, the ground surface will freeze from time to time. By this reason, a heating unit of a large capacity and/or an underground piping for circulation of warm water is usually necessary to install at the greenhouse site. In order to keep the inside of the greenhouse warm, a large size duct for warm air has to be mounted on the ground surface so as to be opened at a suitable elevation above the ground surface, and provision has to be made of a suction device on the opposite side for drawing warm air into the greenhouse from said duct.
The glass greenhouse so far employed may roughly be divided into a lean-to-roof, three-quarter, saddle-roof and a round-roof type. In the case of the most popular double-roof type, a plurality of equally spaced shape bars are connected to a central ridge beam and to one another by connecting bars for providing a roof portion or structure. The latter is carried by a plurality of mid pillars and a sidewall structure, the ends of which in the direction of the ridge beam are supported on an underground foundation for providing an overall frame structure.
A door for entrance into or exit from the greenhouse is provided on the front side of the greenhouse, and a suitable number of windows may be provided along the ridge beam and/or on the sidewall portion.
Alternatively, a suitable number of equally spaced rigid angle frame units may be provided for forming a skeleton structure for the greenhouse. In this case, it may be necessary to provide suitable reinforcements in both the roof and sidewall portions such as structs generally extending along the ridge beam.
The upper surface of the roof and sidewall portions of the greenhouse thus formed are usually lined for instance by a plastic fiber reinforced plate glass.
With the glass greenhouse of the above-described conventional design with a single plate glass, especially in frigid districts, the inside of the greenhouse cannot be warmed or kept warm sufficiently if only one or more heating units provided within the greenhouse are resorted to. In frigid districts, such as Hokkaido of Japan, the soil may become frozen to some depth below the ground level. Thus an underground pipe must be provided for circulating warm air and/or warm water for melting the frozen soil.
In a certain small size greenhouse such as those lined by a vinyl chloride sheet, only the roof portion is lined by two vinyl chloride sheets, the other portions of the greenhouse being lined with a vinyl single sheet. The basic concept of the inventive greenhouse is that it would be convenient to provide a double wall at the roof and all sides of the glass greenhouse and to utilize the space of the double wall to its best advantage. According to the present invention, there is provided a glass greenhouse in which upper and lower surfaces of the rigid angle frame units constituting the greenhouse skeleton structure are lined by glass plates for providing a roof portion, a sidewall portion a front surface and a rear surface all of which are lined by double glass plates. Within the internal space of the so defined double wall structure, the warm air from the same supply source as that used for warming the inside of the greenhouse is forced to flow for warming and keeping warm the inside of the greenhouse in conjunction with the usual heating by the conventional heating unit provided within the greenhouse.
The greenhouse of the present invention is basically designed for use in frigid districts for growing of cereals, vegetables or fruits. Hence, an underground piping system or network is embedded directly below and around the greenhouse for circulation therethrough of the warm air supplied from the same source.
If the warm air is not forced to flow down the inside of the double wall structure, the air will remain in the inside space of the double wall portion, thus giving rise to the transfer of heat through it and the double glass wall, by virtue of the complex heat transfer phenomenon consisting of heat radiation, convection and conduction. In a well-known manner, the resistance of the air layer to heat transfer does not increase in proportion to the increase in the thickness of the air layer. If the resistance to heat transfer and the thickness of the air layer are plotted on the ordinate and the abscissa, respectively, the resulting curve will be substantially parallel to the abscissa for a thickness of the air layer in excess of a predetermined value. The reason for this is that the heat transfer through the air layer is caused not only through heat conduction, but through heat convection and radiation, and especially heat convection will become a predominant factor with the increase in the air layer thickness. In the conventional double roof structure of the small size greenhouse, referred to above, a layer of asbestos or similar heat insulating material has been filled into the space of the double wall for compensation of insufficient heat insulating properties of the air layer. Such an artifice is naturally undesirable because the heat insulating layer will prevent the sunlight from entering into the greenhouse.
According to the greenhouse device of the present invention, described above, the warm air from the same source as that used for delivery of warm air into the inside of the greenhouse is forced to flow down the inside of the double wall. Thus the heat which may otherwise be transferred between the inner and outer walls of the double wall structure is absorbed by the warm air for positively preventing the heat transfer through convection from taking place. The warm air supplied to the double wall structure constitutes a warm air curtain serving itself as heat insulating layer. If, owing to the temperature difference between the inside and outside of the double wall portion, the wall surface has been cooled to lower than the dew point, with resulting dew formation on the wall surface, such moisture will be entrained in the warm air flow and hence no cloud will be generated on the glass surface.
The total amount of the warm air to be supplied to the double wall structure can be calculated by obtaining the heat flow per unit area through the two glass plates and the air layer therebetween, multiplying the heat flow rate thus obtained by the total surface area of the double wall structure for obtaining the heat flow, and dividing the latter by the enthalpy of the warm air, as will be described below.
The warm air which has been cooled after routing through the inside of the double wall structure and the underground piping system is returned to the entrance to the warm air generating unit or to the air conditioner at the exit side of the generating unit. In either cases, the air so returned is passed through a measuring unit for measuring the amounts of oxygen and toxic gases and added with oxygen as the occasion may demand. The air is then routed to the inside of the living and control space or the greenhouse, or to the inside of the double wall and the underground piping network for circulation, bypassing the conditioned air generator.
The warm air, which has been cooled more or less after its routing through the inside of the double wall structure, is recirculated through the warm air generating and conditioning unit which forms part of the inventive greenhouse device. The air so circulated may be used for mixing with the hot air in place of the outside air, depending on the prevailing outside air temperature, as will be discussed in more detail.
The temperature of the warm air may be in the range of 0.degree. to +40.degree. C., depending on the usage of the greenhouse.